1. Field of the Invention
The present invention relates to a separator for fuel cells using a metal plate coated with TiN, a method for manufacturing the same and a polymer electrolyte membrane fuel cell comprising the separator, particularly to a separator for fuel cell using a metal plate coated with TiN, a method for manufacturing the same and a polymer electrolyte membrane fuel cell comprising the separator, wherein the separator exhibits an excellent electrical conductivity, corrosion resistance and firmness, particularly, is thin, light, easily-processed and economically excellent, compared to the prior art using graphite, and has no problem of a lifetime reduction due to the corrosion of metal separator caused by the electrolyte, contrary to the prior art using a metal separator.
2. Description of the Related Art
A fuel cell using a polymer membrane having proton exchange characteristic as an electrolyte includes, for example, a solid polymer electrolyte membrane fuel cell (SPEFC) and a direct methanol fuel cell (DMFC).
Such a polymer electrolyte membrane fuel cell has advantages of a high efficiency, high current density and output density, a short start-up time and a quick response to variations of load.
In particular, since the polymer electrolyte membrane fuel cell uses a polymer membrane as an electrolyte, electrolyte loss is not a concern, a methanol reformer can be employed, which is an established technology, and the cell is less sensitive to pressure variations of reacting gases.
In addition, the polymer electrolyte membrane fuel cell has a simple design and can be easily manufactured and various materials can be used for a main body of the fuel cell. Additionally, the fuel cell has high volumetric and gravimetric power density and can provide various ranges of output, compared to a phosphoric acid fuel cell that is operated by the same principle.
Accordingly, the polymer electrolyte membrane fuel cell can be applied to various fields, such as a power source of a zero emission vehicle, an residential power generator, a power source for a spaceship, a mobile power source and a power source for the military, etc.
The polymer electrolyte membrane fuel cell has a basic structure having a polymer electrolyte membrane, porous cathode and anode, which are covered with a noble metal catalyst, mounted to both sides around the membrane, and a separator installed to an outside of the electrodes.
The electrodes are prepared by applying a mixture of catalyst and a polymer electrolyte, i.e., ionomer to an upper part of porous carbon paper, which is water proof-treated, wherein a coating of a liquid polymer electrolyte is formed on the surface of the catalyst and thus a three-phase interface with platinum catalyst and the electrolyte is formed.
The catalyst layer is generally made of powder of platinum or carbon powders carried with platinum and ruthenium to maximize a surface area thereof. When applying the catalyst, a spraying method, a filtration and deposition method and a screen printing method, etc. are used.
A commercial electrolyte membrane is disposed between the cathode and the anode prepared as described above, and then they are hot-pressed under a certain pressure at a glass transition temperature (Tg) of the electrolyte membrane or higher, thereby providing MEA.
The separator supports the electrodes and distributes the reactant gases. It is also referred to as a bipolar plate or flow field plate. A gas flow field of a fuel electrode is formed on a surface of the separator and a gas flow field of an air electrode is formed on another surface of the separator.
Hydrogen, which is a fuel, enters the gas flow field of the anode and oxygen or air, which is an oxidizer, is introduced into the gas flow filed of the cathode. An electrical energy is generated on the electrodes by an electrochemical oxidation of the fuel gas introduced and an electrochemical reduction of the oxidizer introduced.
In addition, the separator provides flow fields for supplying the fuel and the oxidizer, functions as a current collector conducting electrons produced at the anode to the cathode, and removes water generated during an operation of the cell.
Additionally, the separator is a main body supporting the MEA and allows a stack to be formed. The separator used in the stack removes heat of reaction and thus functions as a cooling water flow filed for maintaining a temperature of the stack constantly.
The cooling water flow filed is formed in all separators used in the stack or only in a part of the separators. The separator having the cooling water flow field formed is supplied with the fuel at one side thereof and the oxidizer at the opposite side thereof, and the cooling water is supplied to the middle of the separator.
Herein, the separator is accomplished by joining two plates, each of which has a gas flow field on a surface thereof and a cooling water flow field on the other surface thereof, with the surfaces having the cooling water flow field being faced each other.
The separator prepared as described above should have a long lifetime and excellent electrical conductivity, corrosion resistance and firmness, and be thin, light, well-processed and inexpensive.
That is, the separator should supply humidified reacting gases, and remove water produced by an electrochemical reaction without fail. In addition, the separator should have corrosion resistance and excellent electrical conductivity for effectively transporting electrons being produced and be light while maintaining the strength.
In addition, the separator should be easily processed and handled and made of an inexpensive material and have a long lifetime for a commercialization. Further, in order to put the polymer electrolyte membrane fuel cell to a practical use, an economic efficiency should be improved through a retrenchment of cost of processing the separator. Further, the power density (power per unit volume) should be improved by reducing a thickness of the separator.
According to the prior art, the separator is mostly made of graphite, which exhibits excellent electrical conductivity and corrosion resistance.
However, since the graphite is fragile, it is difficult to handle the graphite. Accordingly, it is difficult to form the flow fields on the graphite surface by machining. In addition, since the graphite is partly gas-permeable, a certain thickness is required to prevent reacting gases from being mixed, resulting in an increase of a volume of the stack.
Further, in order to commercialize the polymer electrolyte membrane fuel cell, the fabrication cost of the stack should be reduced to 1/100 or less. Currently-used graphite separators occupy about 60% of the stack cost. Accordingly, the application of the graphite to separator is not suitable in the economic efficiency aspect.
Meanwhile, as an alternate material to replace the graphite, carbon composites have been developed by injection-molding of carbon powder and resins. The carbon composite separators are more economical than the graphite separators. However, the carbon composites have disadvantages of low electrical conductivity and low mechanical strength.
As another separator material, metal alloy such as stainless steel can be used. Since metal has an excellent electrical conductivity and mechanical properties, and is easily processed and inexpensive, metal separator may satisfy the characteristics required for the separator. However, the prior metal separator has a problem that lifetime of the separator is short due to the corrosion of the metal.